Obviously Lincoln's election in 1860 immediately launched a string of seceding states. I think this is a special case. Lincoln campaigned on a policy of containment, not abolition. His first inaugural address was conciliatory, annoying some in the North. The point is that de facto, Lincoln was president of the of the Union without the Confederacy. Within the Union there were divisions, but he won reelection in 1864 by a good margin
He tried conciliation, but so did Nixon who is currently in second place. The country was divided over Viet Nam, and Nixon ended it. The draft created demonstrations, etc., he ended it. Racial tension created unrest, he ushered in affirmative action. Environmental activism took off at this time, and he created the EPA. He even tried conciliation with the Commies. So, how was that divisive? To some extent, it's about the man, not policy.
Nixon united the country in at least one way. When he was forced out, no one was complaining. lol